Stars are born from vast dust clouds such as the Orion Nebula (pictured). But why such clouds do not collapse immediately into stars under the weight of their own gravity has been something of a puzzle. Astronomers have theorized that magnetic fields help prevent such collapse.

A new study has put this idea to observational test. The result, says University of Illinois astronomer Richard M. Crutcher, is to "complicate the theory."

Crutcher analyzed measurements of the magnetic fields of 27 interstellar dust clouds, including the Orion Nebula. A weak magnetic field permeates the galaxy. It is generally stronger where matter exists, such as in stars or planets. As the dust clouds become more concentrated, their magnetic fields intensify.

By comparing the magnetic energy of each cloud with its gravitational energy, Crutcher determined that magnetism is responsible for about half of the outward pressure that prevents gravitational collapse. Explaining the mechanism involved, Crutcher compares the magnetic fields to rubber bands that "try to straighten themselves out" as they become curled. The fields have a "minimum energy state," which consists of straight lines of force.

Turbulence within the clouds is believed to provide the remainder of the resistance to gravitational collapse. Such turbulence arises from various causes, including shock waves produced by supernovae, and the movements of the galaxy's spiral arms.

"Magnetic fields are important but not dominant," says Crutcher, who reported his findings in the August issue of the Astrophysical Journal. In the past, Crutcher notes, some theoretical models of star formation have downplayed the effect of magnetism, while other models have regarded magnetism as an extremely important factor. The new findings point toward a "blend" of such theories...